The present invention relates to an image data processing method for separating a target object having a preselected size from bidimensional image data input via a radar or a sonar, and an apparatus therefor. More particularly, the present invention is concerned with a new image data processing method capable of extracting a target object having a preselected size even from data having a low S/N (Signal-to-Noise) ratio or involving background noise, and an apparatus therefor.
It is a common practice with a sonar image processing apparatus to separate a target by removing a reverberation image from an underwater acoustic image in which the target exists in reverberations. To improve the image quality of the screen, it has been customary to use a threshold scheme, as taught in, e.g., Japanese Utility Model Laid-Open Publication No.S58-110869 and Japanese Patent Laid-Open Publication No.H1-201182. As shown in FIG. 11 of the drawings specifically, the threshold scheme determines whether or not signals or echoes received by a sonar have levels higher than a preselected threshold level and displays only signals whose levels are higher than the threshold level on a sonar screen. However, the conventional threshold scheme has some problems left unsolved, as follows.
It is necessary for the operator to adjust the threshold manually while watching a sonar image screen. This is troublesome particularly when the level of echoes from a target object and the level of background noise or that of reverberations are extremely close to each other, i.e., S/N ratio or S/R ratio is low. Specifically, in such a condition, the operator must adjust the threshold to an extremely narrow range shown in FIG. 12 specifically. The adjustment therefore needs expertness.
Further, assume a specific sonar image screen in which the general receipt level varies with respect to distance or time as shown in FIG. 13. Then, if the threshold is adjusted to, e.g., TH1 in order to separate a target object T1, then the image of a target object T2 is lost. Moreover, the receipt level distribution sometimes varies in a single sonar image, depending on the azimuth of receipt. This is likely to discard necessary information when a single threshold is applied.
It is therefore an object of the present invention to provide an image data processing method capable of separating the image of a target object existing in background noise and reverberations without requiring the operator to perform threshold adjustment, and an apparatus for practicing the same
It is another object of the present invention to provide an image data processing method capable of detecting all of a plurality of target objects even in a single image in which the distribution of receipt levels varies, and an apparatus for practicing the same.
In accordance with the present invention, there is provided an image data processing method for processing data of a bidimensional image to extract a target from the bidimensional image, the bidimensional image including pixels arranged in horizontal and vertical directions, the data being representative of gradation of each of said pixels, comprising: (a) sequentially connecting, in a bidimensional connecting area having a preselected size, data of pixels in a vertical or horizontal direction to produce a gradation change signal; (b) executing FFT (Fast Fourier Transform) with the gradation change signal to thereby generate frequency harmonics components representative of the gradation change signal in frequency domain; (c) selecting predetermined harmonics components from the frequency harmonics components, the predetermined harmonics components corresponding to size of the target. Further, the image data processing method according to the present invention comprises steps of (d) executing IFFT (Inverse Fast Fourier Transform) with the selected predetermined harmonics components into time domain to reproduce image data of the target and (e) correcting levels of the selected predetermined harmonics components to be supplied to the step (d) in accordance with a DC component in the FFT result.
Also, in accordance with the present invention, there is provided an image data processing apparatus for processing data of a bidimensional image to detect a predetermined sized object, the bidimensinal image having pixels arranged in horizontal and vertical directions, the data being representative of gradation of each of said pixels, comprising: a memory for storing data of a bidimensional image; means for sequentially reading the gradation of each pixel in a vertical or horizontal direction in a connecting bidimensional area designated in the bidimensional image to produce a gradation change signal, the connecting bidimensional area being determined in accordance with size of the predetermined sized object; transforming means for FFT transforming the gradation change signal into frequency domain to deliver a plurality of frequency components; and selecting means for selecting desired frequency components from the plurality of frequency components transformed by the transforming means, the desired frequency components corresponding to size of the predetermined sized object. Further, the image data processing apparatus according to the present invention comprises IFFT means for IFFT transforming the desired frequency components into time domain to reproduce image data of the predetermined sized object and correcting means for correcting levels of the desired frequency components to be supplied to the IFFT means in accordance with a DC component in the FFT result.